Author’s Note: This paper will be helpful for those who would like to delve more deeply and broadly into the geology that has shaped both the Highlands-Cashiers Plateau and the wider Southern Appalachians of which it is a part.  It was originally prepared as a term paper for a course entitled “Geology of the Southern Appalachians,” taught in 2020 at UNC Asheville by Dr. Jackie Langille.  While I have modified it to include more definitions and explanatory material than were in the original, its form, tone and language are still more academic than those in Whence These Special Places?  So it’s probably not a good starting point for the totally uninitiated.  However, I expect that those who have grasped the principal concepts of Whence These Special Places? will have the framework needed to find this paper both informative and helpful in furthering their understanding and appreciation of the processes revealed in the rocks of our region.  

For useful graphics about the geologic provinces and geologic ages referenced in this paper, click on the <Appalachian Geologic Provinces> and <Geologic Timescale> buttons on the Research & Resources page.

A Geologic History of the Southern Appalachians

Bill Jacobs, May 2020

 The Blue Ridge is a composite of polydeformed metasedimentary
crystalline terranes that represent both the early Paleozoic Laurentian
margin and accreted terranes.”

                                                                            

                                                                                Arthur J. Merschat, 2009

Introduction.  Arthur Merschat’s summary of Blue Ridge geology, written in full-on doctoral-dissertation style, highlights key elements of the development of the Blue Ridge province – “polydeformed,” “metasedimentary crystalline terranes,” “Laurentian margin,” and “accreted terranes.” This paper will explore those elements as they apply within the Blue Ridge, and also relate their underlying tectonic processes and related depositional histories to the broader Southern Appalachians, to include the Valley and Ridge, Plateau and Piedmont provinces. The discussion will be organized chronologically, but will weave in the elements highlighted by Merschat as they emerge at various points in the process.

First, however, some definitions for the terms used by Merschat. “Polydeformed” means that the rocks of the Blue Ridge have experienced multiple rounds of deformation, such as bending, folding, and squeezing into new patterns and layering. “Metasedimentary” means that the rocks were formed from sedimentary accumulations (as opposed to igneous processes involving molten magma), and that they were subsequently subjected to sufficient heat and pressure (typically by deep burial) to be metamorphosed into different rocks. “Crystalline” refers to rocks that have formed directly from minerals crystalizing out of cooling magma, or whose crystal structure has been modified by metamorphic processes (that is, they are not sedimentary rocks). “Terrane” is used by geologists to designate a region whose geology reflects a distinctive history different from that of surrounding areas, and implies a chunk of crust that has been imported (“accreted”) during tectonic processes. “Laurentia” refers to the continental landmass we today know as North America, and its “margin” is the edge created as Rodinia divided and “Gondwana” (which contained today’s Africa and South America) rifted away during the opening of the Iapetus Ocean; the margin is thus the region most affected by new seashores, flooding of nearby low-lying landscapes (and thus deposition of mud and sand that formed new rock layers), and the accretion of crust when renewed tectonic activity pushed terranes and seafloor material onto the continental landmass.

The Middle (or “Meso”) Proterozoic. Although the mountains now exposed in the Southern Appalachians were formed much later than the Middle Proterozoic, it was during that era that tectonic forces created and emplaced the bedrock of today’s eastern North American continent, upon which the current mountains were built. The large-scale event was the formation of the supercontinent of Rodinia, approximately 1.2 – 1.0 GYA. Hatcher (2005). As plate boundaries were subjected to compressive forces, ocean basins closed, sedimentary rocks were emplaced on the continent, and mountains were raised in what is known as the Grenvillian “orogeny” (geologist-speak for a mountain-building event).  As sedimentary layers were buried by this process, they would have experienced the first of the metamorphic crystallization and deformation events that can be identified today in the Southern Appalachians.

Mesoproterozoic rocks form the region’s oldest exposures, but they are not widely visible at the surface. In general, they are found as highly metamorphosed gneisses along a SW – NW swath 5 – 15 kms wide and about 200 kms long, running from north of the Great Smokies into Virginia, a few miles west of Asheville and mostly just to the east of the Tennessee border. Clark (2008), Merschat, C. & Cattanach (2008). Elsewhere in the region, these early metasedimentary rocks were not detached and transported with younger overlying rocks during the much-later Alleghanian orogeny, or they at least remained more deeply buried during the transport process. In addition, a substantial, heavily metamorphosed pluton associated with the Grenvillian orogeny, and now known as the Toxaway Gneiss, is exposed along the Blue Ridge Escarpment southwest of Brevard.

The Most Recent (or “Neo”) Proterozoic. Rodinia first began rifting apart around 735 MYA, but in the area occupied by today’s Southern Appalachians, the early phases of rifting did not immediately result in the creation of a new continental margin. Hatcher (2005). Rather, what is sometimes referred to as a “failed rifting” process caused the crust to thin, resulting in volcanic activity reflected in rocks exposed in the Mt. Rogers area and dated to approximately 760 MYA. Merschat, A. et al (2016). Another effect of the rifting process was the creation of inland basins that filled with sediments that lithified as deep layers of sandstones and shales. Buried and metamorphosed by subsequent orogenies, and caught up in the great northwestern transport of the Blue Ridge-Piedmont Megathrust Sheet during the Alleghanian orogeny, the rocks of one of these basins now appear as gneisses and quartzites along the western edge of the Blue Ridge in North Carolina and far eastern Tennessee. Bearing the general name of the Ocoee Supergroup, their leading edge forms much of the Great Smokies. Clark (undated).

Rodinia completed rifting about 565 MYA, forming what today is roughly the eastern boundary of the North American plate, with the creation of ocean basins between Gondwana  and Laurentia. This process set the stage for the rocks seen today in much of Southern Appalachia, in several ways. First, the ocean basin closest to Laurentia, known as the Iapetus Ocean, filled with deep layers of sediments flowing off the Laurentian uplands. Second, as Gondwana and Laurentia separated, significant land masses appear to have been severed from both Laurentia and Gondwana, becoming large islands between Gondwana and Laurentia. These events created the raw material for crystallized, polydeformed, metasedimentary rocks exposed in multiple terranes. Their conversion into today’s Blue Ridge involved tectonic transformations and transport during a 300 – 350 million-year period stretching through much of the Paleozoic.

The Paleozoic (541 – 252 MYA). The processes that transformed the Neoproterozoic raw materials arose from the creation of a new supercontinent, named Pangea, as Gondwana and Laurentia, as well as the islands in between, again came together as one large, contiguous landmass. The broad process is referred to as the Appalachian orogeny, but it was divided into three distinct orogenic episodes.  Because they occurred along the converging margins of the Gondwanan and Laurentian plates, they all are examples of what geologists would call “convergent tectonics.”

The Taconic Orogeny. This first stage of Paleozoic convergent mountain-building began during the Cambrian and continued until approximately the end of the Ordovician, about 440 MYA. Known as the Taconic orogeny, it involved eastward subduction of the leading, oceanic edge of the Laurentian plate beneath a plate approaching from the southeast. (Note – all plate directional references are based on North America’s modern orientation). Hatcher (2005). As the dense, hydrous oceanic crust subducted, the heat and release of moisture generated magmas that rose to the surface to create an arc of volcanic islands, as is seen today in subduction zones around the Pacific Ocean basin. In addition, as the overlying plate approached Laurentia, it created an accretionary wedge of the mixed sedimentary rocks of the Iapetus Ocean basin, emplacing them on the Laurentian landmass. These rocks were buried to depths of 15 km or more, where metamorphic processes converted them into the crystalline rocks today known as the Ashe Metamorphic Suite (AMS)/Tallulah Falls(TF)/Alligator Back formations. It seems likely that much of the burial was at least in part under the now-eroded islands of the volcanic arc, which would have come ashore as the Iapetus Ocean closed. In any event, these rocks now form much of the eastern third of the Blue Ridge from north Georgia, across North Carolina, and into Virginia.

Among the polydeformed crystalline formations found in the Blue Ridge are a group of terranes located immediately to the west of the AMS/TF in southwestern North Carolina. These include the Dahlonega gold belt, Cartoogechaye and Cowrock terranes, which collectively form the Central Blue Ridge, divided by major faults from the AMS/TF-dominated Eastern Blue Ridge and the Rodinian-dominated Western Blue Ridge. Merschat, A. (2009). These formations have not been fully interpreted, but from their predominately metasedimentary character, they seem likely to have originated as chunks of the Laurentian crust or margin. Hatcher (2010). Presumably severed during the rifting of Rodinia, they would have been brought back together with the continental landmass as the Iapetus Ocean closed. Because they are located to the west of AMS/TF, it seems likely that they were emplaced at an early stage of the accretionary process; the Cartoogechaye terrane was especially deeply buried, experiencing the highest metamorphic conditions recorded in the Taconic. Hatcher (2010).

The Taconic orogeny also resulted in the creation of batholith-scale plutons, likely a mix of magmas resulting from fractional melting due to burial of crustal rocks at great depth, and possibly from subduction-related moisture releases. Both the Whiteside pluton, running for 30 kms or so to the northeast from Highlands, NC, and the Henderson gneiss, exposed in a broad 100 km-long swath along the western edge of the Piedmont from South Carolina near the Georgia line past the Lake Lure area in North Carolina, Clark (2008), have been dated to 450 – 465 MYA, which would have been the latter stages of the Taconic. Several smaller plutons along the Blue Ridge Escarpment in South Carolina (the Caesar’s Head and Table Rock gneisses) have also been dated to the late Taconic. Jubb (2010).

The Neoacadian Orogeny. The geologic record contains little, if any, evidence of tectonic activity for some 75 million years after the Taconic orogeny drew to a close at the end of the Ordovician. The next orogenic surge affecting Laurentia appears to have begun in the Devonian and continued into the early Mississippian, and is best known by its effects to the east and northeast rather than in the Southern Appalachians. Named the Acadian orogeny to the northeast, the somewhat more recent phase that affected the Southern Appalachians is typically called the Neoacadian. This event is responsible for implanting the Carolina superterrane in today’s North Carolina Piedmont. This landmass, as well as others to the northeast, were probably originally associated with Gondwana, and accreted as the oceans between Gondwana and Laurentia continued to close. Hatcher (2010). As it docked, it brought along an intervening terrane, the Cat Square, which also forms part of today’s Inner Piedmont. Merschat, A. (2009). Interestingly, it appears that the Cat Square originated 200 – 300 km to the northeast and was transported into the North Carolina Piedmont by “channel flow” of ductile (viscous) material beneath the approaching Carolina superterrane, in part along the Brevard Fault Zone, Merschat, A. (2009), a process that potentially was extended by the early-Alleghanian transform collisional forces discussed below. Jubb (2010).

Although called an orogeny for its effects farther to the north, and a major emplacement event in the Piedmont, the Acadian/Neoacadian convergent events did not generate significant mountains in the Southern Appalachian region. Stewart & Roberson (2007). However, careful geological research has identified numerous less dramatic effects of the Neoacadian within the Blue Ridge. These include: a lengthy period of renewed metamorphism, particularly in the Inner Piedmont, Merschat, A. (2009); contributions to polydeformation, in the form of small-scale folding that overprints Taconic deformation, Jubb (2010); and transform faulting along the Burnsville fault, Stachowitz et al (2019). In addition, a scattering of granitoid intrusions have been dated to the Devonian, particularly the Spruce Pine pegmatites and Pink Beds pluton. Hatcher (2005), Jubb (2010).

The Alleghanian Orogeny. The climactic phase of the convergent tectonics that created Pangea and shaped today’s Southern Appalachians is known as the Alleghanian orogeny. Beginning as the Neoacadian came to a close 330 – 320 MYA, it lasted 30 – 40 million years through the Pennsylvanian into the middle of the Permian period, and culminated with the collision/joinder of Gondwana and the eastern margin of Laurentia. Recent interpretations posit something of a north-to-south rolling collision of (using modern names) the west African prominence with eastern North America, with zippered closure from the north, substantial transform pressure in today’s Piedmont, and eventual direct southeast-to-northwest impact with the southeastern North American coast. Hatcher (2010).

Based on modern-day parallels, and on metamorphic levels reflected in rocks at today’s surface, the orogeny created a Himalayan-scale range of mountains, stretching 3,000 kms from Alabama to Newfoundland. Hatcher (2010). At the latitudes of the Southern Appalachians, these mountains were centered to the east in today’s Piedmont, where their heavily metamorphosed roots are now exposed.

Within the Blue Ridge province, the orogeny’s effects were somewhat less direct. Rather than piling mountains on top of the terranes emplaced during the Taconic (and thereby causing a further round of high-level metamorphism), the collision resulted in the detachment and northwestern transport on broad thrust faults of a large, multi-layered crustal sheet known as the Blue Ridge-Piedmont Megathrust Sheet. This thrust structure extended from north Georgia across the Carolinas and into southern New York, Hatcher (2010), and involved transport distances estimated at over 300 kms. Hatcher (2005). Its effect was to push older rocks from the Taconic and the Grenvillian basement over and across younger rocks formed in shallow seas west of the Taconic-era mountains. The basal thrust surface is visible today as the Linville Fault within the Grandfather Mountain Window; also, younger sedimentary rocks, surrounded by mountains formed from older crystalline rocks, can be seen at Cade’s Cove and similar windows along the western edge of the Blue Ridge.

While the en masse nature of the Alleghanian transport process generally preserved the Taconic spatial relationships among the Eastern and Central Blue Ridge terranes and the more westerly formations of the Laurentian margin, the southern portion of the principal fault system separating the Eastern and Central Blue Ridge (the Chattahoochee/Burnsville/Holland Mountain/Gossen Lead system), was at least reactivated, as evidenced by the Chattahoochee cross-cutting the early-Alleghanian Rabun pluton.  Merschat, A. (2009). Moreover, the northwestward pressures left their mark within the Blue Ridge, particularly in fold structures at hand-sample to landscape scales, with a strong SW-to-NE regional strike – another, and probably dominant, Jubb (2010), round in Merschat’s polydeformational count. As also occurred during the earlier orogenies, the earliest stages of the Alleghanian left a legacy of notable igneous intrusions, including the Rabun pluton mentioned above, as well as the Looking Glass. Jubb (2010).

The Alleghanian orogeny is also responsible for shaping two other geologic provinces associated with the Southern Appalachians, the Valley and Ridge and, further to the northwest, the Appalachian Plateaus. These provinces consist of younger, layered sedimentary rocks deposited on a Precambrian base in shallow inland seas following both the rifting of Rodinia and the development of hinterland mountains during the Taconic. In the Valley and Ridge, the northwestward pressure created large-scale folds, striking SW to NE, across a 25 – 50 km swath stretching 1,500 kms into New York. Hatcher (2005). Reflecting variations over time in water depth, the sedimentary layers alternate among sandstones, limestones and shales. The varying resistance of their upturned edges has resulted in ridges dominated by resistant sandstones, and valleys that have been cut into the less resistant limestones and shales, all in parallel alignment with a distinct SW-to-NE strike. Clark (undated).

Farther to the west, a broad area of eroded uplands known as the Cumberland and Appalachian Plateaus stretches from Alabama across central Tennessee northeastward into Pennsylvania. The Plateaus consist of the same young, horizontally layered and undeformed sedimentary rocks found to the west, but are over 1,000′ higher in elevation than those more westerly areas.  Their distinctive elevation is generally explained as a product of the orogenic processes creating the Appalachians, but without the folding and faulting found in the Valley and Ridge, which was closer to the impact zone.  However, this explanation somewhat oversimplifies the process.  A more nuanced approach would credit the Paleozoic orogenies with providing the raw materials for the Plateaus, in the form of sediments eroded off the mountains and ridges formed to the east as Gondwana collided with Laurentia.  But the actual lifting does not appear to have been along the types of thrust faults that raised (and distorted) rocks of the Valley and Ridge, but rather was part of the much-more-recent Cenozoic uplift shared with the broader Appalachians and described below.  US National Park Service (undated).

It should be noted that while the Plateaus’ elevated terrain can be rugged, this is an effect of modern streams cutting into an elevated landscape, and not of differential uplift or folded layering.

The Mesozoic (252 – 66 MYA) and Cenozoic (66 MYA to present).  Following the assembly of Pangea, a process that was completed approximately 265 MYA, tectonic activity affecting the Southern Appalachians largely ceased for about 50 million years. Hatcher (2005). It then resumed in the form of rifting, with today’s Africa separating from North America on a 200 million-year march to the southwest, creating not only separate continents but also the Atlantic Ocean. While Africa took with it a portion of the great Alleghanian mountain range, it left a legacy of metamorphosed crystalline terranes, elevated thrust sheets, and complexly folded structures, all of which began to rapidly erode as the low-lying ocean basin opened to the east.

In recent decades, geologists have explored a serious problem with the easy and popular concept that today’s Southern Appalachians are the result of uninterrupted diffusive erosion of the larger mountain ranges formed during the Paleozoic orogenies. The problem is that expected rates of upland erosion would have leveled the Alleghanian mountains within 50 – 100 million years, far less than the 300 million years since their original formation, or the 200 million years since the rifting of Pangea would have accelerated their erosion. It thus seems that later uplift, or “rejuvenation,” must have occurred. Evidence that this is the case is now accumulating. Some is observational, such as recent erosion reflected in the steepness of V-shaped mountain valleys, and the sinuous paths of such major rivers as the French Broad (which would have developed sinuosity in broad low river bottoms, not in valleys descending from high mountains, but could have maintained the sinuosity in gorges cut during slow uplift of the established streambed). Evidence also includes studies: of knickpoints in relatively young V-shaped valleys, Gallen (2012), and other signs of topographic disequilibrium; of east-west lineaments in an otherwise SW-to-NE-striking landscape; of a mismatch between geologic provinces and points of highest elevation; and of resurgent sedimentary deposition on the Appalachian flanks, Hatcher (oral comments, 2019). Generally, Hill (2020).

While there is growing acceptance of regional uplift during the mid-Cenozoic, the process is not understood. No evidence exists of convergent tectonic activity (and related faults and thrust sheets). Isostatic rebound, as the crust adjusts to the removal of mass due to either the erosion of high mountains or melting of glaciers, can create uplift; however, neither of those sources of rebound can explain the Southern Appalachians, which are well west of the Piedmont where the burden of the highest mountains was removed, and well south of significant glacial accumulations. Possible candidate mechanisms related to the mantle include upward pressure from a large area of unusually high mantle temperatures, and circulation effects of the remnants of the Farallon Plate as it completes its eastward subduction under North America from the Pacific coast. The latter hypothesis seems, however, to suffer from lack of any similar effect as those remnants crossed under the Great Plains and Mississippi Valley. Another possibility that appeals to this author is delamination of a large swath of dense, eclogite-rich material from the base of the crust, enabling the relatively lighter remaining crust to isostatically rise. Hill (2020). A zippered event of this sort, beginning in the south and proceeding to the north, could explain both why the Southern Appalachians are today the highest and widest portion of the overall Appalachian structure, and why the unusual lineaments are oriented east-to-west rather than radially as would be expected with an isochronous event creating a single dome.

Regardless of the cause of Cenozoic uplift, the rugged topography of today’s Southern Appalachians is best understood as the interaction of such uplift and renewed erosion as higher elevations increased the erosive power of water flowing off the fresh uplands.  It is also in part a product of higher rainfalls associated with Gulf air masses being forced over the region’s renewed elevations. This type of energetic erosion accounts for the steep slopes of many of the region’s ridges, which also often have sharp tops. As a general matter, erosion would have proceeded at a faster pace along the existing sinuous rivers as they cut gorges through the rising landscape (such as the French Broad, and quite likely the sinuous Little Tennessee, Pigeon and Tuckasegee systems), and also in areas where the bedrock has extensive pre-existing fractures (such as the principal valleys in the Highlands-Cashiers area, which frequently are located along the axial planes of long-eroded AMS anticlines). On the other hand, slower erosion would be expected in harder rocks, such as the quartzites of the Black Mountains and metasandstones of the high ridges of the Great Smokies, Clark (undated), and the sandstones frequently found along the ridges of the Valley and Ridge province. Slower erosion would also be expected in less deeply fractured rocks, such as the plutons that form great rock walls and exfoliation domes in the Eastern Blue Ridge, or that underlie the high valleys of the Highlands-Cashiers Plateau (Jacobs, 2019).

 

References and Bibliography

Clark, Sandra H.B., 2008, Geology of the Southern Appalachian Mountains: US Geological Survey.

Clark, Sandra H.B., undated, Birth of the Mountains: US Geological Survey.

Gallen, Sean F. et al, 2012, Miocene rejuvenation of topographic relief in the southern Appalachians: GSA Today, V. 23, no 2.

Hatcher, Robert D., Jr., 2005, Southern and Central Appalachians: in compendium of articles on North American geology, Elsevier.

Hatcher, Robert D. Jr., 2010, The Appalachian orogen: A brief summary: The Geological Society of America, Memoir 206.

Hill, Jesse S., 2020, Not old dead mountains, but slumbering giants; Cenozoic uplift of the southern Appalachians: UNCA lecture series, March 6, 2020.

Jacobs, William S., 2019, Whence These Special Places? The Geology of Cashiers, Highlands & Panthertown Valley: Great Rock Press.

Jubb, Mary Grace Varnell, 2010, Paradoxes in the deformational and metamorphic history of the eastern Blue Ridge …: available through the Tennessee Research and Creative Exchange (Trace).

Merschat, Arthur J., 2009, Assembling the Blue Ridge and Inner Piedmont: Insights Into the Nature and Timing of Terrane Accretion in the Southern Appalachian Orogen …: available through the Tennessee Research and Creative Exchange (Trace).

Merschat, Arthur J., Southworth, Scott, et al, 2016, Geology of the Mt. Rogers area, Revisited: Carolina Geological Society 2016 Annual Field Trip Guidebook.

Merschat, Carl E. and Cattanach, Bart L. (2008), Bedrock Geologic Map of the Western Half of the Asheville 1:100,000 Scale Quadrangle, North Carolina and Tennessee:NC Geological Survey, Geologic Map Series – 13.

 National Park Service (undated), Physiographic Provinces Series, Appalachian Plateaus Province, https://www.nps.gov/articles/appalachiannplateausprovince.htm

Stachowitz, Liana, Stith, Felix and Langille, Jackie, 2019, Bedrock Geologic Map of the Clyde 7.5-minute Quadrangle, western North Carolina: UNCA, Technical Report USGS EDMAP Grant G18AC00099.

Stewart, Kevin G. and Roberson, Mary-Russell, 2007, Exploring the Geology of the Carolinas: The University of North Carolina Press.